In order to satisfy the demand for petroleum and its products, tertiary recovery processes have been developed in which quantities of petroleum remaining in reservoirs after water flooding may be recovered by the use of chemical flooding. For instance, it is known to drive microemulsions through a reservoir with subsequently introduced high viscosity polymeric solutions. In this case, the term "microemulsion" defines a thermodynamically stable emulsion containing oil, water and a surfactant. Such processes are described in U.S. Pat. Nos. 3,506,070 and 3,506,071 which are incorporated herein by reference. It is also known to formulate the microemulsion within the reservoir through injection of an aqueous surfactant solution which then combines with the petroleum and water found within the reservoir.
However, these known processes suffer a number of deficiencies. The first mentioned process has the disadvantage that the surfactant used is ionic and generally in admixture with further co-surfactants. In long-lasting underground petroleum recovery, an alteration of the uniformity of the distribution of the surfactants within the microemulsion occurs resulting in a non-homogeneous emulsion which does not function efficiently. Further, the efficiency of this microemulsion also substantially depends on the salt concentration of the water component prevailing in the reservoir.
The second process has the disadvantage that the microemulsion is formed within the reservoir only after the injection of the aqueous solution of surfactants. Here too, ionic surfactants are usually used in combination with one or more cosurfactants. Therefore, the resulting variations in the composition and the efficiency of the resultant microemulsion are not unexpected. In particular, variations in the salt content of the water component of the reservoir have a considerable influence with respect to the variations in the efficiency of the emulsion.
These recovery methods also suffer from the disadvantage that the microemulsion which is injected into or formed within the reservoir has to be driven through the reservoir by means of a polymer solution. It has been found that extraordinarily large amounts of polymer solution are necessary for this purpose thereby greatly increasing the cost of the process. Finally, it must also be noted that the salinity of the reservoir fluids must fall within a required range. If this salinity is not present within the reservoir, it may be necessary to correct it through injection of water having the desired salt concentration.
Recently, there was proposed a process according to which a chemical flooding process can be carried out even if the salt concentration of the reservoir is above the optimal concentration. This process necessitates a graduated salt concentration to be introduced into the reservoir by means of the surfactant and polymer solutions and requires that the reservoir minerals be relatively insensitive to low salt contents.
The problem to be solved therefore is to provide an improved process for the emulsion flooding of petroleum reservoirs at an optimum efficiency independent from the salt concentration or its variations within the water phase of the reservoir, without the necessity of using a mixture of surfactants of different chemical composition which might result in an inhomogeneity during production and without the necessity of driving the microemulsion through the reservoir with the aid of a polymer solution.